A terminal unit is usually mounted in a suitable opening in an external wall of a building. Known forms of terminal unit are usually complex and expensive to construct because they contain a multiplicity of baffles and/or other components. For this reason they tend to be bulky structures which protrude beyond the outer face of the wall to an undesirable extent--in some cases by as much as 10-15 cms. This projection can result in an unsightly outward appearance. Moreover the presence of baffles and/or other components can, in some cases, impose undue resistance to the flow of combustion products away from the unit.